Plate tectonics, long-distance dispersals and chance introductions can explain many trans-Pacific disjunctions

Several diatom species, including Cymbella amplificata, C. janischii and Navicula aurora, are largely restricted to Asia and western North America.These disjunctions likely represent recent long-distance dispersals or chance introductions.The two continents also support clusters of diatom species in several fossil and extant genera.One of these genera, Gomphosinica, is exclusive to the two continents but includes only one species in common.This suggests that these disjunct clusters of species arose from common precursors that underwent radiations while being separated for millennia.The divergence of these lineages likely began more than 25 Ma when island arcs of Asian (Siberian) origin docked on North America.The Asian/North American connection is not restricted to diatoms; vascular plants and dinosaur fossils also exhibit this connection.Differences between diatom floras east and west of the Rocky Mountains, as first noted by Ehrenberg, can be explained by differences in Earth history and geology.Researchers interested in diatom biogeography would do well to become familiar with the tectonic history and geology of their geographic regions of interest.Some cases of 'invasive' or 'alien' diatoms may simply be that changing conditions allowed existing but latent species to prosper and become apparent to the unaided eye.